A boy who is turning 12 years old wants to read sci fi and fantasy with strong females. We have suggestions, of course.
Ann McCaffrey is often suggested because she’s a woman who writes science fiction and therefore must be feminist. Turns out that’s not true. She’s a VERY BAD CHOICE as she promotes rape culture. (Third book, protagonist rapes his girlfriend, and makes everything ok by helping her clean afterward. No. No. No. No. I stopped reading her after that book, but I am told that her later books have similar or worse issues with rape.) (Thanks to #2’s warning, #1 hasn’t read that series, but I haven’t found anything problematic in the McCaffrey I have read. #2 notes that’s probably because McCaffrey coauthors with actual feminists in many of her other series.)
Tamora Pierce in general and in specific, though #2 has a bit of a problem with the fourth book in her first series… the main character has a lot of messed up sex, and by messed up, she means messed up in terms of power differentials. The sex is not really consensual given the power differences in one “relationship” and the age differences in the other. (#1 missed that series. The Pierce I have read has been fantastic!) Holly Lisle probably has too much sex for a 12-year old. Pre-read Diplomacy of Wolves to see if your kid is old enough for it.
Robin McKinley (her lighter stuff… Deerskin [a retelling of Donkeyskin] is feminist, and amazingly good, but it contains rather violent incest… Sunshine has happy sex in it IIRC, but is definitely more YA than Junior). The Blue Sword was the first grown-up fantasy book I ever read (fourth grade assigned reading, I LOVED Mrs. A.) and it got me hooked on the entire genre.
Patricia C. Wrede, particularly the first two books in the Dragon series (Dealing with Dragons is the start). The third and fourth books leave the protagonist somewhat helpless until a boy grows up to save the day.
Joan Aiken’s Dido Twite series: first is The Wolves of Willoughby Chase.
Nina Kiriki Hoffman. Though she does address some adult themes, they always have happened off-stage before the book starts. The characters heal during the book.
For hard sci-fi, you could start the Honor Harrington series with On Basilisk Station by David Weber. Jane Yolen’s graphic novel Foiled is a must (the sequel is Curses! Foiled Again). Anything by Susan Cooper, though #2 notes that The Dark is Rising has a stereotypical female character, the stereotypical “male” action coming from the boys.
You could try Diane Duane’s series starting with So You Want to Be a Wizard. Everything by Diana Wynne Jones is very excellent, though her last book has an inappropriate sexual relationship thrown in as an afterthought. A 12 year old might not notice it. (#1 didn’t.)
The Ruby in the Smoke by Philip Pullman. Only sort of fantasy, but has sequels if you like it.
The Blossom Culp series by Richard Peck (time travel, ghosts, etc. put this fully into spec fic!)
Lloyd Alexander’s Vesper Holly series, starting with The Illyrian Adventure (spec fic in the sense that Indiana Jones is spec fic).
The Scarlet Pimpernel by Baroness Orczy. (Sure, this is historical, but… you could pretend it’s fantasy.)
Graceling by Kristin Cashore is excellent (though somewhat hardcore, violence-wise). It has sequels but I never read them.
Other good YA stuff is by Zilpha Keatley Snyder (any of it).
I can go on if you want… but I would need to check DC’s bookshelves for all my old YA books. (I’m totally going to read hir The Real Me at some point, though that is not science fiction or fantasy at all.)
Chime in, readers!